Investigation and Optimization of Residue-Free Plasma-Assisted Reflow Soldering of SnAgCu by DoE

The goal of the investigations was the development of a technology for residue-free, plasma assisted reflow soldering of SnAgCu solders. Based on the already existing knowledge of plasma assisted reflow soldering (Herzog, 2002), the investigations on the use of Sn3.0Ag0.5Cu alloys and their wetting promotion were accomplished by means of plasma activation and passivation for different assembly surfaces of electronic manufacturing. The low pressure plasma pre-treatment with specific cleaning and passivation of the bonding surfaces facilitates thereby the realization of particularly residue-free reflow soldering processes. Due to the increasing integration of electrical, optical and micromechanical functions into the electronic products the question of the optimal electronic packaging and interconnection technique arises frequently. The conventional reflow soldering technology, with the use of wet-chemical flux agents and their remaining residuals after the soldering process, can hardly fulfil all requirements by in the future. Owing to cost considerations in the industry, it is difficult to achieve a good compatibility between electrical, optical and micro-sensory functions using the present manufacturing methods. The plasma assisted reflow soldering offers itself here as a good alternative. It\´s a special residue-free soldering procedure, which exhibits a large application potential. The practical experiments were carried out for the Sn3.0Ag0.5Cu solder. Based on today\´s knowledge level, this solder alloy is considered to be one of the most frequently used lead-free solders in electronic manufacturing in the future. The effect of plasma activation on different metal surfaces was observed and compared. The degree of the surface activation was determined on the basis of the wetting contact angle and the wetting surface area of the plasma-treated solder. These solders were eventually melted at 240degC. A "design of experiments" was developed with the different plasma- parameters as inputs and wetting contact angle and wetting surface area as outputs, and the above-mentioned experiments were carried out. The optimal plasma parameters required for minimum wetting contact angle and maximum wetting surface area were thus obtained